Polysaccharide

Polysaccharides are compounds made up of many monosaccharides linked together by glycosidic bonds. They are the most abundant type of carbohydrate found in food. Their structures can range from simple linear chains to highly branched polymers. Examples include storage polysaccharides like starch, glycogen, and galactogen, as well as structural polysaccharides such as hemicellulose and chitin. The term "glycan" is a synonym for polysaccharide, though it is often used when discussing glycoconjugates—hybrids of polysaccharides with proteins or lipids.

Polysaccharides are frequently heterogeneous, with slight variations in their repeating units. They can be amorphous, like starch, or insoluble in water, like cellulose. While simple sugars (monosaccharides) have the general formula (CH₂O)ₙ (where n is three or more), polysaccharides have the formula Cₓ(H₂O)ᵧ, with x and y typically between 200 and 2500. When the repeating units are six-carbon monosaccharides, the formula simplifies to (C₆H₁₀O₅)ₙ. Polysaccharides are often highly branched. For instance, starch’s helical portions can trap iodine molecules, producing a dark blue color. Cellulose lacks these complex helices, so it does not bind iodine and shows no color change—a unique test for distinguishing polysaccharides.

As a general rule, polysaccharides contain more than ten monosaccharide units, while oligosaccharides contain three to ten; the exact cutoff varies by convention. Polysaccharides are important biological polymers, typically serving structural or storage functions in living organisms. Starch, a glucose polymer, is a storage polysaccharide in plants, composed of linear amylose and branched amylopectin. In animals, the analogous glucose polymer is glycogen, which is more densely branched and sometimes called "animal starch." Glycogen’s structure allows faster metabolism, suiting the active lifestyles of moving animals. In bacteria, polysaccharides play a key role in multicellularity.

Cellulose and chitin are structural polysaccharides. Cellulose, found in plant cell walls and other organisms, is the most abundant organic molecule on Earth. It is used in paper and textiles, and as a feedstock for rayon, cellulose acetate, celluloid, and nitrocellulose. Chitin has a similar structure but includes nitrogen-containing side branches, making it stronger. It is found in arthropod exoskeletons and fungal cell walls, and is used in products like surgical threads. Other polysaccharides include callose, laminarin, chrysolaminarin, xylan, arabinoxylan, mannan, fucoidan, and galactomannan.

Nutritional polysaccharides are common energy sources. Many organisms easily break down starches into glucose, but few can metabolize cellulose. Some bacteria and protists can, and ruminants and termites use microorganisms to process it. Some polysaccharides are not very digestible but serve as dietary fiber, enhancing digestion. Soluble fiber binds bile acids in the small intestine, reducing their absorption and lowering blood cholesterol. It also slows sugar absorption, reduces post-meal blood sugar spikes, normalizes blood lipids, and, when fermented in the colon, produces short-chain fatty acids with wide-ranging physiological effects. Insoluble fiber is linked to reduced diabetes risk, though the mechanism is unknown. Dietary fiber is considered important, and many developed countries recommend increasing fiber intake.

Starch is a glucose polymer with alpha-linkages, made up of 15–20% amylose (a linear chain of several hundred glucose units) and 80–85% amylopectin (branched, with thousands of glucose units, each branch containing 24–30 units). Starches are insoluble in water and can be digested by breaking alpha-linkages. Humans and other animals have amylases for this purpose. Major dietary sources include potato, rice, wheat, and maize. Starches are how plants store glucose.

Glycogen serves as secondary long-term energy storage in animal and fungal cells, with primary energy stored in adipose tissue. It is mainly produced by the liver and muscles, but also by glycogenesis in the brain and stomach. Glycogen is similar to starch, sometimes called animal starch, with a structure like amylopectin but more branched and compact. It consists of α(1→4) glycosidic bonds with α(1→6)-linked branches. Glycogen is found as granules in the cytosol or cytoplasm of many cell types and plays an important role in energy metabolism.